1. Field of the Invention
The present invention relates to a package for housing a photosemiconductor device, and more particularly, to a package for housing a photosemiconductor device having an improved lens member fixing structure.
2. Description of the Related Art
A typical conventional package for housing a photosemiconductor device comprises, as illustrated in the cross-sectional view of FIG. 5, a substrate 21 made of a metal such as an iron-nickel-cobalt or copper-tungsten alloy, having a mounting portion 21a on which the photosemiconductor device 24 is to be mounted via a Peltier device 25, on the center of a top side of the substrate 21, the substrate having a plurality of external lead terminals 26 which are fixed around the mounting portion 21a to penetrate the substrate 21 from the top side to the bottom side thereof via an insulator 27; a frame 22 made of a metal such as an iron-nickel-cobalt alloy, which frame 22 is attached onto the substrate 21 so as to surround the mounting portion 21a, and has a through hole 22a in a side portion thereof; a cylindrical metallic fixing member 29 made of a metal such as an iron-nickel-cobalt alloy, which is fixed in the through hole 22a of the frame 22 so that an inner end portion protrudes inside the frame 22, and in an outer end portion of which an optical fiber 30 for optical signal exchange between the photosemiconductor device 24 and an external device is to be fixedly inserted, a cylindrical lens 31 fixed in the inner end portion of the metallic fixing member 29 and hermetically separating the inside and outside of the frame 22; and a lid 23 to be bonded to the top surface of the frame 22 and hermetically sealing the photosemiconductor device 24. To obtain a photosemiconductor apparatus as a product, the photosemiconductor device 24 is bonded onto the mounting portion 21a of the substrate 21, the electrodes of the photosemiconductor device 24 are electrically connected to the external lead terminals 26 via bonding wires 28, the lid 23 is bonded to the top surface of the frame 22 so that the photosemiconductor device 24 is hermetically housed in a package mainly comprising the substrate 21, the frame 22 and the lid 23, and the optical fiber 30 is inserted and fixed in the metallic fixing member 29.
Such a photosemiconductor apparatus functions as a photosemiconductor apparatus used for high-speed optical communications in such a manner that the photosemiconductor device 24 excites light in response to a drive signal supplied from an external electric circuit, and the excited light is received by the optical fiber 30 through the cylindrical lens 31 and is transmitted through the optical fiber 30.
The cylindrical lens 31 is made of a glass material which has a lens function achieved by making changes in refractive index from the central axis to the circumferencial portion thereof. To fix the cylindrical lens 31 to the metallic fixing member 29, a metal thin film of, for example, Ti.Pt.Au is applied onto the entire circumferential surface of the cylindrical lens 31, the cylindrical lens 31 with the metal thin film applied thereto is inserted into the metallic fixing member 29, and the metal thin film applied onto the circumferential surface of the cylindrical lens 31 and the inner surface of the metallic fixing member 29 are brazed with a brazing filler metal of a gold-tin or gold-copper alloy.
However, in the package for housing a photosemiconductor device, since the thermal expansion coefficient of the iron-nickel-cobalt alloy of which the metallic fixing member 29 is made (approximately 6.times.10.sup.-6 /.degree. C.) largely differs from the thermal expansion coefficient of the glass of which the cylindrical lens 31 is made (approximately 10.times.10.sup.-6 /.degree. C.) and the entire circumferential surface of the cylindrical lens 31 is brazed, large thermal stress is caused between the cylindrical lens 31 and the metallic fixing member 29 because of the thermal expansion coefficient difference therebetween when the circumferential surface of the cylindrical lens 31 is brazed to the inner surface of the metallic fixing member 29 with a brazing filler metal such as a gold-tin alloy, and the thermal stress directly acts on the edges between the circumferential surface and the end surface of the cylindrical lens 31. Consequently, since stress is apt to concentrate on edges, the stress largely concentrates on the edges and stays therein. When heat generated during operation of the photosemiconductor device 24 is repetitively applied thereto, the thermal stress causes in the cylindrical lens 31 cracks starting at the edges between the circumferential surface and the end surface of the cylindrical lens 31 in synergy with the stress staying in the edges. Consequently, when the light excited by the photosemiconductor device 24 is transmitted to the optical fiber 30 through the cylindrical lens 31, irregular reflection occurs at the cylindrical lens 31, so that the light excited by the photosemiconductor device 24 cannot be excellently received by the optical fiber 30 through the cylindrical lens 31, or the hermeticity of the package is broken by the cracks caused in the cylindrical lens 31. As a result, the photosemiconductor device 24 housed in the package cannot be normally and stably operated for a long period of time.